The present embodiments relate to a potting compound.
Large-volume gradient coils of a magnetic resonance device may include three coil sections for generating magnetic field gradients in the three spatial directions (X, Y, Z). The X- and Y-coils are designed as saddle coils, and the Z-coil is implemented by a peripheral winding. The individual coils may be constructed as bundled individual conductors. Separating structures may be developed in an electrically conductive plate (e.g., made from copper or aluminum) using a suitable process, and the remaining material may form the coil winding. The coil windings produced according to the various processes are connected with an electrically insulating support plate and, in a formative act, are formed, for example, as a semi-cylindrical casing. The individual coil layers are mounted in succession on a cylindrical mandrel. Further components of the coil assembly may be insulating and reinforcement layers, one or more cooling layers (e.g., including plastic tubing, through which a coolant such as water flows) and shim coils. Further layers forming the coil assembly are, for example, secondary windings that shield the magnetic field generated by the primary coils from the outside.
The complete coil assembly is encapsulated in a casting resin, for example, based on epoxy resin. All spaces between the conductors are impregnated without cavities or bubbles. The casting resin, which may be a potting compound, has a broad range of properties. These include, for example, a low viscosity during processing, so that all spaces between the conductors are completely impregnated (e.g., free from cavities or bubbles), and a high modulus of elasticity in order to provide a high overall rigidity and thus more accurate positioning of the individual windings. The casting resin may have good thermal conductivity to provide effective transfer of heat from the conductor structures to the cooling layer. The casting resin may have a high heat resistance that is reflected in a high glass transition temperature so that as constant a property profile as possible may be achieved in the operating temperature range. The casting resin may have a low coefficient of thermal expansion (e.g., if possible, similar to the coefficient of thermal expansion of the other materials used (copper conductors, insulation layers)) in order to prevent mechanical stress and thereby a simplified crack formation, which may lead to cracks and peeling in the coil unit when heated both during operation and during the cooling from the curing temperature. In this context, a high crack resistance that is manifest in the form of a high critical stress intensity factor KIc combined with a high critical fracture energy GIc, should also be mentioned. A high partial discharge resistance, a low dielectric loss factor, flame retardance, and economic aspects should also be mentioned.
Thermally curing epoxy-based casting resins may be used as potting compound for, for example, large-volume coils. This potting compound may contain approximately 65% filler by weight (e.g., in the form of quartz powder, aluminum oxide or wollastonite microparticles). Microparticles provide that the particle size is measured in micrometers. EP 1 850 145 A2 discloses a potting compound or a magnetic coil cast with a potting compound. This potting compound includes microparticular fillers and inorganic nanoparticles. A potting compound with such a composition has very good crack resistance based on the combination of a very high critical stress intensity factor K−c with a very high critical fracture energy GIc. The fillers used effect some positive changes to the cured potting compound, for example, with regard to heat resistance, crack resistance, thermal conductivity and economic aspects. It would therefore be desirable to implement as high a filler content as possible. There are no specifications included with regard to flame retardance. The potting compound is not a flame-resistant, nanoparticular potting compound.
High filler content causes the flow behavior of the prepared potting compound to be considerably impaired. Furthermore, the type of filler has a significant impact on an actual increase in crack resistance. This is worsened, for example, due to the aluminum oxide trihydrate Al(OH)3 (abbreviated to ATH) that is frequently added for flame retardance reasons. An increase in the glass transition temperature of the base resin mixture that serves as the supporting matrix also causes a worsening of the crack resistance. Fillers tend toward sedimentation or filtration, for example, on the glass fabric layers used for reinforcement. A specific potting resin composition is therefore a compromise between the required properties.